Success of Hybrid Fracs in the Basin

Abstract

Abstract In recent years waterfracs have increased in popularity, providing relatively long fracture lengths without the polymer damage of conventional crosslinked gelled fracture treatments. The major disadvantages of waterfracs are proppant settling and bridging, which limits waterfracs to high pump rates and low sand concentrations. These disadvantages gave way to the creation of the hybrid fracture stimulation, which is a combination of a waterfrac prepad followed by a crosslinked gelled fluid. This paper will compare the use of hybrid fracture treatments in the deep, high pressure, high temperature, tight gas formations of the Anadarko Basin in Oklahoma to the more conventional waterfrac and crosslink frac. Also the fracture geometry, cooling effects and cost savings of the hybrid will be discussed. Introduction The term "hybrid" has been used to describe several different types of fracture stimulations consisting of various combinations of slickwater, linear gelled and crosslinked gelled fluid systems.1 For the purpose of this paper a hybrid frac is in reference to a stimulation fracture treatment consisting of a waterfrac prepad followed by a crosslinked gelled fluid. It is also important to note that the term waterfrac and slickwater frac are used interchangeably to describe a fluid consisting of water, either fresh or 2 wt% potassium chloride (KCl) in combination with a friction reducer. For several years operators have used hybrid fracs in the Anadarko Basin. Hybrids have been successfully pumped in various formations of the Anadarko Basin ranging from the Marmaton (10,800 feet) all the way down to the Atoka A (15,000 feet). The waterfrac portion of the hybrid fracs typically ranges from 45 to 60 percent of the total fluid pumped. 2 wt% KCL water is pumped with a friction reducer at 0.5 to 1.0 gallon per one thousand gallons of fluid. The crosslinked portion typically consists of water gelled with 25 to 30 pounds of a guar based polymer per one thousand gallons of fluid (ppt). Hybrids vs Crosslinked Fracs When dealing with the tight gas formations of the Anadarko Basin, long thin fractures are preferable over short wide fractures.2 With the use of hybrid stimulation treatments the initial fracture is created with the slickwater prepad, creating a long thin fracture. The crosslinked gelled fluid will follow the path of least resistance. The width and height of the fracture will increase with the introduction of the crosslinked gelled fluid into the formation but the increase does not correspond to the width and height that would be generated with a conventional crosslinked gelled fluid fracture treatment. To illustrate the differences in fracture geometry Meyers 3-D Fracture Modeling was used to model two fractures on a twelve-foot Red Fork zone located in Roger Mills County, Oklahoma. A representative openhole log section of the zone is illustrated in Fig. 1. First, a conventional crosslinked gelled fluid fracture treatment was simulated using 40,000 gallons of fluid and 90,000 pounds of proppant pumped at a rate of 35 barrels per minute (bpm). The fluid used was a crosslinked 30 ppt gelled water. Based on a 0.5 pounds per square foot proppant concentration per fracture area cutoff the theoretical geometry of one fracture wing was found to be 699 feet long, 115 feet high and have an average width of 0.0855 inches. The graphical output from the simulation illustrating the proppant coverage of one wing is shown in Fig. 2. Next, a hybrid treatment was simulated utilizing 40,000 gallons of slickwater prepad in place of 6,000 gallons of the pad utilized in the previous simulation. The rest of the fluid and pumping schedule was the same. Using the same cutoff criteria as above the theoretical geometry was found to be 783 feet long, 101 feet high and have an average width of 0.0796 inches. Fig. 3 illustrates the proppant coverage of this simulation over one wing.